1. Field of the Invention
This invention relates to geological structural modeling of subsurface rock formations based on well log data.
2. Background Art
Information about formation dips is important in oilfield exploration. For example, dip information may be used to determine the locations of particular zones (e.g., shale zone, sand zone, etc.) within a formation. This information may be used to determine whether or how a well can be drilled in an appropriate formation.
Formation dips may be measured on a small scale (i.e., a few centimeters) or on a large scale (i.e., tens of meters). The measurement of dips on a small scale may be performed with resistivity-type well logging tools, such as a Fullbore Formation Microlmager (FMI™) tool, a Dipmeter tool, etc. The measurement of dips on a large scale may be performed using seismic equipment. Multiple well logs from one or more tools may be required to determine dip angles or other information related to dipping planes for a particular formation.
For example, dipmeters may be used to make high resolution micro-resistivity measurements around the borehole circumference, which may then be correlated to produce dip information. This information may be merged with tool orientation/navigation data to provide information on formation dips in the earth's frame of reference.
Once a dipmeter tool has traversed depths of a well, or an area of interest within the well, a plurality of resistivity logs are produced. By properly correlating the fluctuations of these resistivity logs, the positioning of a dipping plane relative to the tool position can be readily calculated.
For example, FIG. 1A shows a schematic illustrating a dipping plane 2 intercepting a borehole 4 at a dipping angle. A horizontal plane 6 is illustrated for comparison. The intercept between the dipping plane 2 and the borehole 4 is an ellipse. When the borehole image is rolled out into a 2D graph, the ellipse manifests itself as a sine curve (FIG. 1B). By such or similar analysis, apparent dip information (relative to the borehole or tool axis) may be derived.
Then, by measuring the bearing of the tool relative to some azimuthal reference, such as magnetic north, and the inclination of the tool relative to the true vertical or gravitational axis, the position of a dipping plane relative to the north and true vertical axes can be determined. To obtain an accurate dip angle, correlations of a number of signals may be necessary.
Various techniques for analyzing formation dips and formation modeling are known in the art. For example, U.S. Pat. No. 4,357,660, issued to Hepp, discloses methods and apparatus for processing measurements indicative of dips and azimuths of formation features in a borehole to produce three-dimensional representations of the formation features. U.S. Pat. No. 4,414,656, issued to Hepp, discloses a well logging system using the output of a dipmeter tool to produce a map showing various characteristics of the earth formations surrounding a borehole. U.S. Pat. No. 5,388,044, issued to Hepp, discloses a method of dipmeter processing that fits a thickness conserving mathematical model to a folded or faulted subsurface sedimentary geological structure to produce dip profiles.
Other disclosures related to dip analysis and modeling can be found in, for example: Etchecopar, A., Bonnetain, J. L., “Cross sections from dipmeter data,” AAPG bull, V. 76, N0.5, Ppp. 621-637 (1992); Etchecopar, A., “Diptrend: Method and automatic programs for geological analysis of a dip file,” Proceedings of the wireline, testing, & Seismic Interpretation Symposium (1991); and Etchecopar, A., Dubas, M. O., “Automatic method for geological interpretation of Dips,” Proceeding of the SPWLA meeting. (Oklahoma City, Okla., 1992).
While these prior art methods are useful in dip analysis and formation modeling, there is still a need for systems and methods that can provide better models for formation dip analysis, especially models having dip information projected away from the wellbore into nearby formation.